Offshore tower apparatus and method

ABSTRACT

A tower suitable for use in offshore well operations and the like including a plurality of sloping jacket legs extending from the bed of the body of water to a position above the surface of the body of water for supporting a platform thereupon. The jacket legs are reinforced by a surrounding shell of diamond patterned cross braces and a plurality of girder rings lying in a plurality of planes normally with the central axis of the tower. The girder rings are supported against deformity by a bicycle spoke reinforcing system at each girder ring level. The method aspects of the invention include constructing the tower in a generally horizontal posture upon a plurality of generally upright columns. The construction steps include forming a plurality of girder rings and erecting the girder rings upon the columns. Jacket legs are connected between adjacent girder rings along the length of the offshore tower and the tower legs are enclosed within an outer shell of cross bracings. The offshore tower, following construction, is launched into a body of water for transportation to a selected marine site by constructing the tower longitudinally upon a rail having one end thereof lying adjacent a sheet pile wall which permits the lower end of the rail to be positioned below the adjacent water level. A floatation system connected to the tower and the wall is removed to permit the base of the tower to be buoyantly lifted from the construction support. The upper portion of the tower rests upon a rail bearing guide bracket which is initially positioned above the water level. The rail bearing guide bracket may be lifted off the rail by an incompressible fluid and the tower slides into the body of water. Alternatively, the tower may be jacked into the water by conventional jacking devices. Upon being erected at an offshore location, conductors may serve in a dual capacity as conductors and piles, or piles may be inserted into skirt pile casings surrounding the base of the tower and driven into the bed of the body of water by a stinger guided by a rotating truss.

United States Patent 11 1 Koehler 1 Nov. 4, 1975 OFFSHORE TOWERAPPARATUS AND METHOD [75] Inventor: Albert M. Koehler, Houston, Tex.

[73] Assignee: Brown & Root, Inc., Houston, Tex.

[22] Filed: Jan. 30, 1974 [21] Appl. No.: 438,013

Related US. Application Data [62] Division of Ser. Nos. 30,098, April20, 1970, Pat. No. 3,668,876, and Ser. No. 238,167, March 27, 1972, Pat.No. 3,815,371.

[52] US. Cl 52/745; 61/465 [51] Int. CLE04G 21/14; E04l-l 12/00; E21B15/00 [58] Field of Search 52/50, 747, 745; 244/125;

Primary Examiner.lames L. Ridgill, Jr. Attorney, Agent, or Firm.lames E.Cockfield [57] ABSTRACT A tower suitable for use in offshore welloperations and the like including a plurality of sloping jacket legsextending from the bed of the body of water to a position above thesurface of the body of water for supporting a platform thereupon. Thejacket legs are reinforced by a surrounding shell of diamond patternedcross braces and a plurality of girder rings lying in a plurality ofplanes normally with the central axis of the tower. The girder rings aresupported against deformity by a bicycle spoke reinforcing system ateach girder ring level.

The method aspects of the invention include constructing the tower in agenerally horizontal posture upon a plurality of generally uprightcolumns. The construction steps include forming a plurality of girderrings and erecting the girder rings upon the columns. Jacket legs areconnected between adjacent girder rings along the length of the offshoretower and the tower legs are enclosed within an outer shell of crossbracings. The offshore tower, following construction, is launched into abody of water for transportation to a selected marine site byconstructing the tower longitudinally upon a rail having one end thereoflying adjacent a sheet pile wall which permits the lower end of the railto be positioned below the adjacent water level. A floatation systemconnected to the tower and the wall is removed to permit the base of thetower to be buoyantly lifted from the construction support. The upperportion of the tower rests upon a rail bearing guide bracket which isinitially positioned above the water level. The rail bearing guidebracket may be lifted off the rail by an incompressible fluid and thetower slides into the body of water. Alternatively, the tower may bejacked into the water by conventional jacking devices. Upon beingerected at an offshore location, conductors may serve in a dual capacityas conductors and piles, or piles may be inserted into skirt pilecasings surrounding the base of the tower and driven into the bed of thebody of water by a stinger guided by a rotating truss.

4 Claims, 41 Drawing Figures U.S. Patent Nov.4, 1975 Sheetlof 103,916,594

US. Patent Nov. 4, 1975 Sheet 2 of 10 3,916,594

US. Patent Nov. 4, 1975 Sheet3of 10 3,916,594

U.S. Patent Nov. 4, 1975 Sheet40f 10 3,916,594

Sheet 5 0f 10 3,916,594

US. Patent Nov. 4, 1975 US. Patent Nov. 4, 1975 Sheet60f 10 3,916,594

US. Patent Nov. 4, 1975 Sheet9of 10 3,916,594

US. Patent Nov. 4, 1975 Sheet 10 of 10- 3,916,594

OFFSHORE TOWER APPARATUS AND METHOD This application is a division ofU.S. application Ser. No. 30,098, filed Apr. 20, 1970, now Pat. No.3,668,876, and application Ser. No. 238,167, filed Mar. 27, 1972, nowPat. No. 3,815,371.

BACKGROUND OF THE INVENTION This invention relates to an offshore towerof the type adapted to be positioned within a body of water such as, forexample, a lake, sea or ocean.

More particularly, the invention relates to an improved offshore tower,method of constructing the offshore tower, method of launching anoffshore tower into a body of water and a method of fixing the offshoretower to the bed of a body of water.

Towers have a multiplicity of applications in a marine environment, suchas for example supports for radar or sonar stations, light beacons,marine exploration labs and the like. Additionally, offshore towers arefrequently utilized in the oil industry in connection with drilling,producing, storing and distributing operations.

Drilling for oil and gas in formations situated beneath the surface of abody of water has in the recent past become an extremely challenging andimportant segment of activity in the oil industry. In this connection,creative scientists and engineers have made tremendous strides inconnection with exploration, drilling, producing, storing anddistributing activities in a marine environment often referred to as thelast earth frontier. Notwithstanding the successes of the recent past,however, significant challenges remain in this infant segment of the oilindustry.

In the initial stages of development, offshore tower operations wereconducted in locations of relatively shallow water depths, from a fewfeet to one or two hundred feet, such as exists along the near shoreportions of the Gulf of Mexico. More recently, however, large mineralresources have been detected in water depths ranging from a few hundredto a few thousand or more feet, such as exists along the Pacific Coastcontinental shelf and the Arctic regions.

In order to exploit mineral resources which exist below such substantialdepths of water, tower designs which have been reliable and effectivelyutilized in the past have undergone considerable redesign for prolongedhigh stress deep water use. In this connection, offshore towerspresently being designed are enormous structures presenting trulysignificant engineering challenges, not only from an initial designaspect, but from a subsequent construction, transportation and erectionpoint of view.

Conventionally, offshore towers are constructed with a plurality ofgenerally upright legs which extend between the bed and the surface ofthe body of water for supporting a platform above the surface of thebody of water. These upright legs are stiffened or reinforced by lateralbrace members or crossing struts. The reinforcement members initiallyrequire accurate cutting operations to form coped ends to intimatelyabut against the circular jacket legs and then require an intricatewelding operation to fixedly connect the bracing to the jacket legs.

While such a structure and technique of fabrication has been generallysatisfactory in the past, significant disadvantages remain. Moreparticularly in dealing with large, heavy structures, it is oftendifficult to provide an exactly coped end portion which will mate with asimilar curved member, particularly in connection with sloping struts.Therefore, excessive welding is often required and in some instances newstruts have to be fabricated. Further, cross braces and struts abuttingagainst the tubular jacket legs tend to punch through the jacket legs orat least deform the jacket legs into a generally eliptical or out ofround configuration. Merely increasing the wall thickness of the jacketlegs is generally unsatisfactory, since the increase in weight of thejacket legs adds significantly to the total weight and cost of theoffshore tower. Further, the legs cannot be adequately reinforced orstiffened internally due to space limitations since the legs frequentlycontain piles, drilling conductors, diver access tubes and the like,which consume the majority of the space in the interior of the tubularleg. Gusset plates and the like have been employed to reinforce theexterior of the legs so that the load transfer takes place across alarger area. However, this indirect transfer causes stressconcentrations which can drastically reduce the fatigue life of thejoint and structure. Further, such additional plates materially add tothe weight of the tower structure, and therefore the overall cost, aspreviously mentioned.

It would therefore be highly desirable to provide a means forreinforcing or stabilizing jacket legs of an offshore tower which wouldtransfer the load through the connection and not into the tower legs.

Further, in at least some instances, it has previously been the practiceto stabilize an offshore tower by forming laterally extending skirt pilecasings about the base of the offshore tower and driving piles, havingapproximately the same diameter as the jacket legs, through the pilecasings and into the bed of the body of water.

In areas, however, where loose bottoms exist or seismic or stronghydrodynamic loads are anticipated, it has been found that suchconventional towers with skirt pile reinforcing have not been totallysatisfactory. In this connection, it would be desirable to provide ajacket piling system which would enhance the capabilities of theoffshore tower to withstand large lateral loads such as produced byshifting earth fonnations, etc.

Another difficulty with previously known offshore towers has been thedifficulty in placing piles within the skirt pile casings and thensubsequently driving the piles deeply into the bed of the body of water.In this connection a plurality of pile driving guide collars have beenestablished at the end of cantilever arms extending along the lateralsurface of the offshore tower at a plurality of elevations with a stringof axially aligned guides for each pile casing. Therefore, following thedriving of one pile the entire driving string has to be raised insegments and reconstructed in the next string of guides. Such a processis extremely laborious, time consuming and economically undesirable.

It would thereofre be desirable to provide a method and apparatus forconveniently guiding the piles into the pile casings and rapidly drivingthe piles deeply into the bed of the body of water without withdrawingthe entire pile driving string following each driven pile.

As previously mentioned, in the early stages of development of theoffshore petroleum industry, drilling took place in shallow waterdepths. Conventional construction equipment and techniques weretherefore capable of fabficating the shallow water towers.

It has been found, however, that in trying to meet the current demandfor deep water structures, which typically may be 500 to 2000 or morefeet in length, conventional construction techniques are often notsuitable to fabricate such enormous towers. In this connection, shipyardfacilities to fabricate these enormous structures presently do notexist. Moreover, once fabricated, there remains the substantial problemof transporting the tower to a body of water for subsequent navigationto a desirable marine site.

It would therefore be highly desirable to provide a method forfabricating large tower structures of indefinite length and acorresponding means of launching the towers thus constructed into a bodyof water for transport to a desired offshore site.

OBJECTS OF THE INVENTION It is therefore a general object of theinvention to provide a method and apparatus which will obviate orminimize problems of the type previously described.

It is a particular object of the invention to provide an offshore towerwith a novel cross bracing system which will minimize the stressconcentrations which typically exist between offshore tower jacket legsand stiffening horizontal braces and struts while simultaneouslyminimizing the total tower weight.

It is a further object of the invention to provide an offshore towerwhich will transfer lateral loads through a cross bracing junction andnot into the tower jacket legs.

It is a further object of the invention to provide a method forfabricating an offshore tower which will not be limited by the size ofthe tower required to be constructed.

It is a still further object of the invention to provide a stabilizingstructure connected to the base of the offshore tower capable ofwithstanding large lateral loads such as produced by seismicdisturbances.

It is another object of the invention to provide a method for guidingand driving piles into jacket pile casings in a convenient and rapidmanner.

It is a still further object of the invention to provide a method forlaunching a deep water offshore tower into a body of water fortransportation to a preselected marine site.

It is yet a further object of the invention to provide a method offabricating an offshore tower segmentally whereby any length may befabricated if desired.

It is yet another object of the invention to provide a method andapparatus for increasing the capacity of a tower to support drillingoperations in a plurality of locations.

It is still another object of the invention to provide a convenientmanner of fabricating an offshore tower which will maintain thegeometrical integrity of the offshore tower during the construction andthe subsequent launching operations.

THE DRAWINGS Other objects and advantages of the present invention willbecome apparent from the following detailed description, taken inconjunction with the accompanying drawings wherein:

FIG. 1 is an isometric view of an offshore tower, with a lower cornerremoved for illustration purposes, positioned within a body of water andresting upon the bed of the body of water with an upper portion thereofextending above the surface for stably supporting a platform withdrilling equipment thereupon;

FIG. 2 is a fragmentary corner view of an offshore tower which in fullembodiment would be illustrated identically as in FIG. 1 with theexception that surrounding the bottom portion of the tower, twice asmany jacket legs are provided which extend alternately from the base ofthe tower upwardly and coextend with twice as many cross bracingconnections to a position intermediate the length of the offshore tower;

FIG. 3 is an isometric view of an offshore tower according to theinvention and is provided with a longitudinally extending pile guidingand driving truss circumferentially riding in the outer periphery of thetower and is further provided with a circumferentially extending belt ofjacket pile casings and piles about the base for pinning the offshoretower to the bed of the body of water;

FIG. 4 is an isometric view of a girder ring forming a portion of theinvention;

FIG. 5 is a segmental detailed view of a portion of the girder ringdisclosed in FIG. 4;

FIG. 6 is a detailed view of a bracket for connecting the girder ringspokes to the hub of the girder ring;

FIG. 7 is a cross-sectional view of FIG. 6 taken along section line 77therein;

FIG. 8 is a detailed segmental view of a portion of a girder ringdisclosing the brackets for connecting the hub spokes with the inner rimof the girder ring;

FIG. 9 is a sectional view of FIG. 8 taken along section line 9-9therein;

FIG. 10 is a sectional view of FIG. 8 taken along section line 10l0therein;

FIG. 11 is a segmental view of a section of the outer periphery of anoffshore tower disclosing the relationship of the tower legs, the crossbracing shell and the girder ring;

FIG. 12 is an isometric view of a segment of an offshore towerdisclosing an alternate girder ring configuration;

FIG. 13 is an isometric view of a segment of the outer portion of anoffshore tower disclosing an alternate connection arrangement of thegirder ring disclosed in FIG. 12, with the tower legs and cross bracingshell;

FIG. 14 is an isometric view of a segment of the outer portion of anoffshore tower disclosing a still further alternate girder ringarrangement;

FIG. 15 is an isometric view of a portion of an offshore towerdisclosing an alternate connection configuration of the girder ringarrangement disclosed in FIG. 14;

FIG. 16 is a plan view of a unitary cross forming a part of theinvention;

FIG. 17 is a top view of the unitary cross disclosed in FIG. 16;

FIG. 18 is a plan view of a segment of a skirt piling casing ring whichmay be deployed around the base of an offshore tower as illustrated inFIG. 3;

FIG. 19 is a sectional view taken along section line 19-l9 of FIG. 18;

FIG. 20 is a detailed view of a connection bridge between a tower jacketleg and a horizontally disposed brace, spanning adjacent skirt pilecasings;

FIG. 21 is a plan view of the bridge shown in FIG. 20;

FIG. 22 is a sectional view of FIG. 21 taken along section line 22-22therein;

FIG. 23 is a detailed elevational view of an alternate connection bridgebetween a tower jacket leg and a horizontally disposed brace spanningadjacent skirt pile casings;

FIG. 24 is a plan view of the bridge shown in FIG. 23;

FIG. 25 is a side elevational view of a pile driving truss asisometrically illustrated in FIG. 3;

FIG. 26 is a side elevational view of the pile driving truss shown inFIG. 25;

FIG. 27 is a cross-sectional view of FIG. 25 taken along section line2727 therein;

FIG. 28 is a detailed view of the truss driving mechanism;

FIG. 29 is a segmental elevational view of a portion of the verticalcolumns utilized to hold the girder rings during the tower constructionoperation;

FIG. 30 is a detailed view of an upper portion of one of the supportcolumns shown in FIG. 29 specifically illustrating the adjustable upperbracket and pillow blocks;

FIG. 31 is a side elevational view of FIG. 30;

FIG. 32 is a cross-sectional view of FIG. 31 taken along section line3232 therein;

FIG. 33 is a schematic elevational view of an offshore tower in apartially completed state of construction;

FIG. 34 is a plan view of an offshore tower positioned within aconstruction bay adjacent a body of water;

FIG. 35 is an end elevational view of the base of the tower asillustrated in FIG. 34, resting upon supporting blocks;

FIG. 36 is an end elevational view of the top of the offshore tower, asillustrated in FIG. 34, resting upon a rail bearing guide;

FIG. 37 is a detailed elevational view of the rail bearing guide;

FIG. 38 is a sectional view of the rail bearing guide of FIG. 37 andtaken along section line 3838 therein; and

FIGS. 39-41 disclose in a sequential schematic array a method oflaunching the previously constructed offshore tower along the monorailof the construction bay and into the body of water for transport to adesired marine site.

DETAILED DESCRIPTION General Structure:

Referring now to the drawings, and more particularly to FIG. 1 thereof,there will be seen an offshore tower 50 situated upon the bed 52 of abody of water 54 and extending above the surface 56 of the body of waterfor stably supporting a platform 58 thereabove. The body of water 54typically may range from 200 to 2000 or more feet in depth. The offshoretower, as previously mentioned, may be used for a multiplicity ofapplications such as, for example, a support for radar stations, lightbeacons, marine exploration labs and the like. More predominantly,however, offshore towers of the type illustrated are utilized in the oilindustry for drilling, storing and distributing operations.

In this connection, the platform 58 frequently is composed of at leasttwo decks, a main deck 60 and a cellar deck (not shown) positionedtherebelow. The main deck may serve to support a plurality of drillingrigs 62 which progressively and simultaneously drill in a plurality oflocations around the periphery of the offshore tower. Further, the maindeck may be provided with a plurality of cranes and various mud tanksand other equipment suitable for sustaining a continuous drillingoperation. The cellar deck typically may contain housing units,generators, compressors, control centers, test facilities and the like.

Tower Structure:

The offshore tower 50 is composed of a plurality of jacket legs 64disposed symmetrically about a central vertical axis 66 and forming anouter tower surface generally in the geometrical configuration of atruncated cone. The peripherally disposed upright jacket legs 64 aresupportingly interconnected by a diamond patterned shell of crossbracings 68 which serve to take lateral loads imposed upon the offshoretower 50. It will be appreciated by those skilled in the art that thediamond patterned shell of cross bracings shown in FIG. 1 in a preferredouter encompasing posture may in some instances be placed within theinterior of the inner tower periphery formed by the jacket legs 64.Surrounding the upright jacket legs 64 and the surrounding shell ofcross bracings 68 are a plurality of girder rings 70 positioned aroundthe outer periphery of the tower.

The girder rings 70 lie in a plurality of mutually parallel planes alllying normally to the central tower axis 66. As will be readily realizedby viewing FIG. 1, the girder rings 70 incrementally diminish indiameter from the base 72 of the offshore tower to the top 74 thereof.Each of the girder rings 70 is supported against out of rounddeformation by a bicycle bracing network 76 (note FIG. 4) which will bemore fully described hereinafter.

The upright jacket legs 64 are columnar structures and are sufficientlydimensioned to receive concentrically within the interior thereof aconductor 78 which is driven into the bed of the body of water 52. Theconductor may be grouted to the interior of the jacket leg and serves toguide a drilling string (not shown) for drilling through the jacket legsinto formations positioned beneath the offshore tower 50. Moreover theconductors serve the at least two additional significant purposes ofstrengthening and supporting the tower.

As illustrated in FIG. 1, it may be desirable in some instances to drillin locations between adjacent jacket legs. In this connection, conductorstrings 80 shown by dotted lines may be guided through a plurality ofaxially aligned conventional funnel shaped collars 82 (note FIG. 8)fixedly connected to the girder rings 70.

In those instances where additional lateral stability is desired, thebase 72 of the offshore tower may be provided with a tighter shell ofcross bracing struts 86 identical in general configuration with thecross bracing shell 68. The spacial area, however, within an individualdiamond is diminished by a factor of four while there are twice as manycrossing junctions at a given planar level. The lateral structuralstrength of the offshore tower is thereby significantly increased.

Referring now to FIG. 2, there will be seen a lower comer segmental viewof an offshore tower 87, the remainder of which being substantiallyidentical with the tower as disclosed in FIG. 1. Tower 87 includes aplurality of jacket legs 88 extending about the periphery of the towerand containing therein conductors 90, which extend coaxially within thejacket legs and deeply into the bed of the body of water. Drillingstrings are then lowered through the conductors 90 for drilling earthformations situated beneath the offshore tower.

The conductor legs 80 are surrounded by a cross bracing shell 92 andgirder rings 94, identically as described in connection with the towerillustrated in FIG. 1.

The offshore tower 87 illustrated in FIG. 2, however, in addition to thestructure of FIG. 1, is provided with a plurality of jacket legs 96which extend between the bed of the body of water, upwardly only apartial distance along the offshore tower lateral surface. Above thejacket legs 96, conventional conductor guide collars 82 (note FIG. 8)are positioned within the girder rings 94 in a manner previouslydiscussed. The lower jacket leg segments 96 serve to isolate the lowerportion of the conductor strings from excessive compressive forces andcurrent stresses which may be produced when the base of the tower ispositioned in relatively deep water. Further, it will be seen thatadditional cross bracings 95 extend from the bed of the body of waterupwardly and coextensively with the jacket legs 96 to increase thelateral structural strength of the offshore tower and distribute loadsto the conductor piles.

Referring now to FIG. 3, there will be seen an isometric view of anoffshore tower 97 again as substantially described in connection withFIG. 1, including an upper platform 98, a plurality of jacket legs 100surrounded by a shell of cross bracing struts 102 and a plurality ofgirder rings 104.

The offshore tower 97 is provided at its base 106 with a plurality ofskirt pile guides 108 extending peripherally about the circumferencethereof. The pile guides 108 are fixedly interconnected with each otherby horizontally disposed braces 110 and sloping struts 112. Piles 114are guided within the skirt pile guides 108 and are driven into the bed52 of the body of water 54 by utilization of a rotating truss 116connected along the periphery of the offshore tower 97 which will bemore fully described hereinafter.

Girder Ring Segment:

The truncated cone offshore towers 50, 87 and 97, as isometricallyillustrated in FIGS. 1-3, all utilize a plurality of girder rings.

As specifically illustrated in FIG. 4, a girder ring 118 is constructedwith a circular outer beam 120 and a coaxially disposed circular innerbeam 122. The beams are interconnected by sloping braces 124 in a mannerwhich will be more. specifically disclosed hereinafter.

The girder ring is supported against out of round deformation by abicycle bracing system 76 comprising an axially disposed tubular hub 126having an upper flange plate 128 and a lower flange plate 130 disposedcircumferentially about the outer periphery of tubular hub 126.Emanating from the upper flange plate 128 and the lower flange plate 130are a plurality of spokes 132. These spokes may be composed, forexample, of a set of steel wires wrapped with a surrounding cloth with aresin covering to prevent excessive corrosion thereof.

The spokes 132 emanate from both the upper and lower surface of both theupper flange plate 128 and the lower flange plate 130. At each junctionlocation of the spokes 132 with the inner beam 122 of the girder ring118, one spoke originates from the upper flange plate 128 and a secondspoke emanates from the lower flange plate 130.

A segment of spoke junction locations have been labelled in the topportion of FIG. 4 as junction points A through E. The spoke lines havebeen hatched in coding to more fully illustrate the connection system.In this connection, reference may be had to the legend at the lowercorner of FIG. 4, wherein the full dotted line indicates a spokeemanating from the lower flange lower side (L.F.L.S. The dashed anddotted line indicates a spoke from the upper flange lower side(U.F.L.S.). Spokes radiating from these flange locations will be seen asconnecting with the beam 122 at position A. In the next clockwiseconnection B, there will be seen a solid line emanating from the upperflange which, as noted in the legend, represents the upper flange upperside (U.F.U.S. while the long dash line represents the lower flangeupper side (L.F.U.S.). Position C on the rim then is provided withspokes (note line coding) from the lower flange lower side and the upperflange lower side. At position D, spokes emanate from the upper flangeupper side and lower flange upper side. Thus, in alternate locationsaround the periphery of the girder ring 1 18 the spokes emanate from theupper flange upper side and lower flange upper side, while in alternatelocations the spokes emanate from the lower flange lower side and upperflange lower side.

Referring now to FIG. 5, there will be seen a detailed view of the hubanchoring structure including the tubular hub 126, upper flange plate128, and the line coded spoke system 76. As specifically illustrated,each individual spoke 132 is formed from a pair of spaced wires 134. Thewires are fixedly connected to the flanges 128 and by a suitable bracketarrangement 136.

The bracket attachments 136 may be formed from one of a number ofconventional designs currently utilized in conjunction with prestressingoperations. One specific embodiment, however, that is satisfactory, isspecifically illustrated in FIGS. 6 and 7. The bracket arrangement 136,as specifically there illustrated, is composed of a base plate 138 and aplurality of normally extending legs 140. A head section 142interconnects the vertical legs and the base plate. The head section 142is provided with a pair of channels 144 which open upwardly and serve toreceive in sliding fashion a flexible wire or braided bundle of strands134. Juxtaposed against the head section 142 of the bracket 136 are oneor more key plates 146 having a pair of downwardly facing channels 148fashioned therein and being dimensioned to spacially conform with theupward channels 144 in the head plate 142 as specifically illustrated inFIG. 7. The wires 134 are formed with integral head beads 150 and arespaced from the key plate 146 by one or more circular bushing rings 152.The key plate 146 in conjunction with the juxtaposed head plate 142serves to confine the ends of the prestressed bundle of wires 134.

Referring now to FIG. 8, there will be seen a segmental plan view of thegirder ring 118 including an outer beam 120 and an inner beam 122. Thebeams are spaced, as previously mentioned, by a plurality of slopingsupports 124 positioned therebetween about their periphery. Further,there will be seen a junction location 154 of the wires 134 forming oneof the spokes 132 of the girder ring arrangement.

As specifically illustrated in FIGS. 9 and 10, it will be seen that theinner rim 122 is composed of a T-beam and is provided with a pair ofinwardly extending parallel channels 156 for the reception of wires 134.A key plate 158 is provided with a pair of inwardly extending compatiblydimensioned channels 160 which serve to retain wire head tabs 162 withinthe channels 156, in a manner previously discussed in connection withFIGS. 6 and 7.

Following the connection operation, the individual wires or bundles ofbraided wire strands 134 are prestressed by conventional hydraulicstressing devices (not shown), and a suitable number of key plates 158or 146 may be inserted to retain the wire in the desired tension. Itwill be readily recognized that such a tensioned spoke system willmaintain the girder ring in an approximately circular posture.Peripheral Tower Elements:

Referring now to FIG. 11, there will be seen an isometric segmental viewof a preferred embodiment of ring girder 1 18 and its manner ofconnection to a plurality of jacket legs 164 and diamond patterned crossbracings 166.

The ring girder 118 is composed of an inner T-beam rim 122 and an outerT-beam rim 120, having the long legs 168 thereof mutually facing eachother and being rigidly interconnected by a plurality of sloping braces124. The braces 124 may be either straps or tubular stock, as preferredor as load requirements dictate. The jacket legs 164 and cross braces166 are positioned between the inner and outer girder rims and arefixedly connected to the inwardly projecting legs 168 through one ormore coped notches 170, fashioned therein as required and one or morehorizontal and vertical tying brackets 172 (note also FIG. 9).

Although mutually facing inner and outer T-beam rims 120 and 122 arepreferred, alternate ring girder structures may be utilized, asspecifically illustrated in FIGS. 12-13 and 14-15.

In the embodiment illustrated in FIGS. 12 and 13, there will be seen afirst alternate embodiment having an outer rim 174 formed from a pair ofT-beam members 176 interconnected by slanting braces 178 and an innerrim 180 formed from a pair of angles 182. The T- beams 176 and angles182, respectively, are interconnected by a plurality of bracing straps184. Both the inner and outer rims 180 and 174 are positioned outside ofthe jacket legs 164 and diagonal cross bracings 166. Interconnectionbetween the ring girder 118 and the jacket legs 164 and the diagonalbracings 166 may be provided by the provision of bridge members 186,extending from the angle members 182 having coped ends 188 to unite, asby welding, integrally with the jacket legs 164 and across bracingmembers 166.

A second alternate embodiment of the ring girder 118 is illustrated inFIGS. 14 and 15. There will be seen an outer rim 189 formed from a pairof spaced T-beam members 190. An inner rim 192 is formed from a pair ofspaced angle members 194. The angles 194 of the inner rim 192 and theT-beams 190 of the outer rim 188 are fixedly interconnected by aplurality of sloping struts 196. The outer rim 189 is interconnectedwith the inner rim 192 by a plurality of brace straps 198. Jacket legs164 and cross braces 166 of an offshore tower are extended between theinner and outer rim members and intimately abut coped projections 200,which extend from the rim segments for uniting contact therewith, suchas by welding.

Cross Bracing Reinforcing Shell:

As previously mentioned in connection with FIGS. 1-3, the outer towerstructure assumes the general geometric configuration of a truncatedcone having jacket legs extending about the outer periphery thereof in asymmetric posture about a central axis of the offshore tower. The jacketlegs are fixedly interconnected by a cross bracing network 66 (noteparticularly FIG. 1).

Referring again now to FIG. 11, there will be seen a detailed view of asegment of the outer tower periphery including jacket leg segments 164and cross bracings 166, fixedly connected thereto. The cross bracingstruts 166 are integrally joined, as by welding, at their junctionlocations by generally hollow unitary crosses 202. The crosses 202 arefixedly connected at the mid points by bridging structures 172 havingcoped ends, as

10 previously mentioned, to the outer periphery of the jacket legs 164.

Referring now specifically to FIGS. 16 and 17, there will be seendetailed views of the unitary cross 202.

The cross is composed of four arms 204, 206, 208 and 210 of uniformlengths and as best illustrated in FIG. 17 are composed of generallyhollow tubular members. The outer ends 212 are formed with normalsurfaces relative to the axis of each arm and serve to abuttingly matewith a similar surface of the strut braces 166. This normal abuttingsurface contact provides a convenient welding junction and serves to uniformly transmit loads through the junction equally around the peripheryof the junction. The entire crossing structure preferably is fabricatedas a unitary piece such as by forging or casting or in the alternativethe inner endsof the legs may be integrally united as by welding.

An angle is formed between adjacent legs with opposing angles alpha, A,and beta, B, being equal. The magnitude of these angles will, bedetermined by the desired slope of the cross bracing arms 166.

Referring now to FIG. 17, the arms 204 and 210 have axes lying insubstantially the same plane 214 and arms 206 and 208 have axes lying insubstantially the same plane 216. Both of these planes are slightlyangled by an amount rho, P, with respect to a plane 218, which liestangent to the outer periphery of the offshore tower. This slight anglepermits the cross 202 to conform to the outer periphery of thecurvilinear tower surface.

The unitary cross 202 is equally dimensioned throughout the entirejacket structure including angles alpha, beta and rho. It will bereadily realized that this unitary cross uniformly positioned throughoutthe outer surface of the tower will transmit axial loads through thejunction locations, as opposed to the previously known technique oftransmitting the loads into the tower jacket legs 164. Further, theunifonn nature of the cross permits mass assembly techniques whichmaterially reduces the time and labor involved in constructing the crossbracing system.

Skirt Pile Guides:

In instances where the offshore tower will be situated upon a softbottom, or the tower must withstand large hydrodynamic or seismic loads,it will be desirable to form a belt of skirt pile guides 108 around theouter periphery of the base of the tower 106. Piles 114 are drivenwithin the guides 108 for pinning the offshore structure to the bed ofthe body of water (note FIG. 3).

Referring now to FIG. 18, there will be seen a top sectional view of asegment of a skirt pile guide structure 220. A plurality of skirt pileguides 108 are horizontally connected by upper and lower tubular bracesegments 110.

As best illustrated in FIGS. 18 and 19, the skirt pile casings 108 areintegrally attached to adjacent jacket legs by longitudinally extendingdiamond patterned bracings 222, which slopingly connect between thejacket legs 100 and the skirt pile casings 108.

Cross bracing struts 166 abut against and are weldingly connected tobridge members 224, as shown in FIG. 18 but more specificallyillustrated in FIGS. 20-22.

The bridge 224 is composed of a pair of horizontal 226 and vertical 228plates fixedly interconnected with a crossing plate 230. The horizontalplates 226 have coped surfaces 232 and the vertical plates 228 have 1 lcoped ends 234 to intimately abut with the adjacent jacket leg I andskirt piling brace 110 for fixed interconnection therewith, as bywelding.

In those instances where the cross bracing arms 166 do not join at theconnection point between the upper and lower horizontal brace arms 110and the jacket legs 100, an I-beam 236, as generally illustrated in FIG.18 and more specifically illustrated in FIGS. 23 and 24, is providedwhich connects between the upper and lower horizontal braces 110 and thejacket legs 100. The I-beams 236 are provided with coped upper and lowersurfaces 238 and coped web surfaces 240 for intimately contacting theadjacent jacket leg 100 and cross brace 1 for being fixedly weldedthereto to unite the pile brace and jacket legs.

It will be noted by referring to FIG. 18 that the cross sectionaldimensions of the skirt pile legs are significantly larger than thecross-sectional dimensions of the jacket legs 100. More specifically,the diameter of the jacket leg 100 is approximately one-half toone-fifth that of the diameter of the pile casing 108. The variation incross-sectional dimensions is provided to increase the capability of thepile casings 108 and piles 114 receivable therein to withstand shear andflexture stresses. More specifically, for a tubular member such as theskirt piles, the load carrying capacity is increased in proportion tothe second power of the cross-sectional dimension. Thus the largediameter piling may withstand significantly larger loads thancorresponding conductor piles while at the same time, the jacket legswhich must be larger than the conductor piles may be maintainedrelatively small through the use of lateral bracing. The resultingstructure is a combination with maximum strength and simultaneously thelong jacket legs with adequate bracing are maintained with relativelysmall dimensions to minimize total structural weight and costs.

Pile Driving Truss:

In order to position the piles 114 within the pile casings 108, a pilepositioning and driving truss 116 (note particularly FIG. 3), issuspended along the lateral surface of an offshore tower.

Referring particularly now to FIGS. 2527, there will be seen detailedviews of the truss 116.

The truss is formed from three generally mutually parallel legsdesignated as type 242 and 243 interconnected by a plurality of struts244 and horizontal braces 246. A plurality of ring girder rollingsupports 248 are connected along the legs 242 of the truss 116 and arespecifically designed to rest upon and roll about the offshore towerring girders.

Referring now to FIG. 28, there will be seen a detailed view of arolling support 248.

The rolling support 248 comprises a channel faced roller 250 adapted torest upon the upper edge of a T- beam 252 forming at least a portion ofthe outer rim of the girder ring. The roller 250 may be driven through agear linkage system which includes a spur gear 254 axially connectedwith the roller 250 and mating with spur gear 256, which may be drivenby a conventional electric or hydraulic motor 258. The roller supportassembly 248 is fixedly connected to one of the legs 242 of the truss116 by a suitable bridge 260.

In operation, the truss is suspended from an offshore tower, such asspecifically illustrated in FIG. 3, and axially aligned with a skirtpile guide 108. A pile may then be axially guided into engagement withthe interior of the skirt pile guide 108. The truss 116 may then be ad-12 vanced about the periphery of the tower until the axis of the trussis in alignment with the axis of the next skirt pile guide 108. A pilemay then be guidingly lowered into the guide and the procedure repeatedcircumferentially around the offshore tower.

A driving stinger may then be lowered within the interior of the truss 116 for driving the piles 114 into the bed of the body of water.Following the driving operation of one pile, the truss 116 is advancedinto axial alignment with the next succeeding pile and the drivingstinger drives the pile into the waterbed. This procedure is duplicatedaround the outer periphery of the offshore tower until all of the piles114 are driven deeply within the bed of the body of water. The truss 116may then be removed for subsequent utilization with other offshoretowers.

It will be appreciated that the rotating truss 116 provides a means forinitially placing and later driving a plurality of piles about the baseof an offshore tower which may be subsequently removed so as not tointerfere with subsequent operations and wherein the piles may be placedand driven without retracting and placing a lengthy pile column witheach shift pile driving location.

Following the driving operation, the piles l14 may be fixedly connectedto the guides 108 as by grouting the interior thereof in a manner suchas specifically described in a United States Hauber et al. Pat. No.3,315,473, assigned to the assignee of this application. The disclosureof this patent is hereby incorporated by reference as though set forthat length.

Method of Fabrication:

As previously mentioned, the subject offshore tower may be constructedin water depths ranging from 200 to 2,000 or more feet. The jacket legsare typically two or more feet in diameter. The overall diameter of thebase of the tower structure, typically may be three hundred or more feetin diameter, while at the water line, the diameter may be I60 feet ormore. When dealing with such large structures, essentially composed oftubular steel members, the construction or fabrication techniquestypically satisfactorily utilized in shipyards for smaller towerstructures are often either economically unsatisfactory or physicallyincapable of performing the construction operation.

A preferred method of constructing the above described offshore towercomprises fabricating a plurality of girder ring segments 118, aspreviously discussed in connection with FIG. 4, in a shipyard.

A plurality of vertically extending columns 270, as illustrated in FIG.29, are then constructed upon a plurality of pilings 272, which extenddeeply into the earth 274 to fixedly support the columns 270. Thecolumns 270 are aligned and correspond in number and spacing to thenumber of tower girder rings required and to normal spacingtherebetween. The height of the columns 270 are designed to beapproximately equal. Positioned at the upper ends thereof are verticallyadjustable supports 276.

The support 276 comprises four upwardly facing outwardly sloping arms278 which connect at their lower ends to a quadraped base 280 comprisingfour upwardly facing members 282 (note FIG. 32) which are interconnectedby horizontal braces 284. The members 282 are adjustably connected tothe upper end of a column 270 by connection with four axially adjustablehydraulic cylinder and ram assemblies 286. At the upper end of the arms278 upwardly extending members 288 13 are longitudinally connected bybraces 290 and serve to support a pair of spaced l-beams 292 which inturn support a pair of pillow blocks 294.

As best illustrated in FIG. 29 the girder rings 118 previously assembledare upended and the hubs 126 thereof positioned upon a pair of adjacentpillow blocks 294 of successive columns 270. The pillow blocks 294 arethen vertically manipulated until the axes of the hubs 126 are inalignment whereupon the hubs are interconnected by spacer tubes 296 intoa column that extends coaxially about the central tower axis.

The girder rings 118 thus suspended in planes being mutually paralleland normal with the central axis of the tower are interconnected by aplurality of generally normally extending jacket leg segments 164, asspecifically illustrated in FIG. 33.

Following the connection of the jacket leg segments 164, the outer shellof truss bracings 166 and the unitary crosses 202 are formed surroundingand reinforcing the jacket legs.

The process of connecting the leg segments, which are axially aligned toproduce generally hollow legs throughout the tower structure, crossbracings and imitary crosses is duplicated throughout the towerstructure until the tower is completed. Method of Launching:

As previously mentioned in connection with the fabrication of theoffshore tower, current tower designs are often enormous structures.Therefore, not only is it a truly significant problem to initiallyfabricate the tower, but the manner of launching the tower into a bodyof water for transport to the desired site presents a significantchallenge.

Referring now to FIG. 34 there will be seen an offshore tower positionedwithin a launching bay 302. The tower 300 may have been fabricated bythe previously discussed technique horizontally upon upright columns270.

In order to launch the tower, a construction bay 302 is first fabricatedalong a shore line where water deep enough to support a large tower ispresent near the shore. The bay 302 is constructed generally normallytoward and into the body of water 306 such that the bay 302 is partiallyon the shore 304 and partially beyond the shore line of the body ofwater 306. The bay 302 is maintained in a dry condition throughout bythe establishment of sheet pile barrier side walls 308 and an end wall309 which permits the lower portion of the tower to be constructed belowthe adjacent water level 306. The columns 270 are positioned along aconcrete rail 310 and the tower 300 is fabricated in a manner previouslydiscussed.

The launching rail 310 is supported upon a plurality of piles 311positioned along the length thereof and driven deeply within the bed 312of the construction bay 302.

Further, in the lower portion of the construction bay, a pair of pads314 are mounted atop a plurality of piles 316 and serve to support bypillow blocks 318 a pair of floatation vessels 320. The floatationvessels are connected to the outer surface of the offshore tower 300 bya bracing system 322 and an internal framing system 323. At the upperend 324 of the tower 300, a toroidal floatation collar 326 isconstructed which at its lower portion is provided with a rail bearingguide assembly 330, which bears upon and is guided along the rail 310 ina manner which will be more fully described herein- 14 after. Thecolumns 270 are then removed and the tower 300 rests upon a three-pointbearing within the construction bay.

Referring now to FIGS. 37 and 38, there will be seen detailed views ofthe rail guide and bearing assembly 330. The toroidal floatation collar326 is provided at its lower portion with a support I beam 332 which hasa downwardly projecting leg ending in a cylindrical bearing rod 334. Thebearing rod 334 rests in a freely pivotal manner upon a rail bearingguide 330.

The rail bearing guide 330 is generally triangular in cross-section asbest illustrated in FIG. 38 having a rectangular bottom surface and apillow block apex 336 for pivotally receiving the cylindrical bearingrod 334. Along the lateral edges of the bearing guide 330 are guide arms338 (note FIG. 37) which serve to asist in maintaining the bearing guide330 upon the rail 310.

The bearing guide is divided by walls 340 which add structural rigidityto the bearing assembly 330. An essentially incompressible fluidsupplied through a piping system (not shown) is directed downwardlytowards the rail in a manner which will be more fully discussedhereinafter.

Referring now to FIGS. 39-41, there will be seen in a generallyschematic array a sequential depiction of launching the offshore towerinto a body of water.

Referring now specifically to FIG. 39, the end wall 309 of thefabrication bay 302 is removed and the body of water 306 enters theconstruction bay seeking its own level. The water lifts the base 342 ofthe offshore tower 300 from supporting contact with the pillow blocks318 due to the buoyancy of the pair of floatation vessels 320. It willbe noted that the spaced relationship of the floatation vessels 320provides, in cooperation with the bearing 330, a three point bearingarrangement which stably supports the tower. As the base 342 of theoffshore tower rises, the upper portion of the tower 324 will pivotabout the bearing rail guide 330.

A source of incompressible fluid 344 is connected into the intereior ofthe bearing guide 330 by a line 346. Incompressible fluid, such as forexample water, oil, soap suds or the like, is then pumped into thechambers formed by the partitions 340. The bearing rail guide 330 isthen lifted from frictional contact with the rail 310. At this juncture,the tower will begin to slide down the sloping rail 310 toward the bodyof water 306. If downward movement does not immediately occur uponunloading of the rail bearing guide, a tug (not shown) may be connectedto the base of the tower to initiate the downward movement of thestructure may be jacked down the rail by conventional jacking devices.The line 346 is of sufficient length to maintain contact with the source340 and the bearing guide 330, for a sufficient period of time for thetower to gain momentum as it descends down the launching rail. Whensufficient momentum is achieved, however, the lubricating fluid is nolonger necessary and the line 346 may be severed. Alternately, theincompressible fluid source may be stored within the tower.

Referring now to FIG. 40, it will be seen that as the tower slides downthe rail 310, the toroidal floatation collar 326 will come into contactwith the surface of the body of water 306 and by its buoyancy lift theupper portion of the tower 324 ofi of bearing contact with the guidebearing 330.

Referring to FIG. 41, it will thus be seen that the offshore tower 300may be completely supported on a floatation system which includes anupper toroidal floatation collar 326 and a pair of spaced floatationvessels 326 connected to the outer periphery of the base of the tower342. The tower 300 may then be towed to a desired offshore site and sunkto the bed of a body of water in a manner more fully disclosed andspecifically claimed in applicants copending US. application Ser. No.29,831, filed Apr. 20, 1975, now US. Pat. No. 3,693,361, issued Sept.26, 1972.

While the specific floatation system utilized in conjunction with thelaunching operation has been described as comprising a toroidal collarand a lower floatation vessel or vessels, it will be realized that otherfloatation systems may beutilized in conjunction with the previouslydescribed launching technique. One such floatation system isspecifically described and claimed in copending US. application Ser. No.29,994, filed Apr. 20, 1970, by Joseph Benton Lawrence, now US. Pat.3,633,369, issued Jan. 11, 1972, said application being of commonassignment with the present application.

In those instances where it is desired to utilize this latter floatationsystem to transport the offshore tower to a desired marine site, it willbe recognized by those skilled in the art that the rail bearing guide330 may be connected directly to an upper brace affixed to the offshoretower 300 and the articulated string of floatation vessels may beattached to the lateral surface of the offshore tower and supported uponpillow blocks substantially similar to pillow blocks 318.

SUMMARY OF THE MAJOR ADVANTAGES It will be appreciated that the abovedisclosed offshore tower is of universal design which may readily beconstructed for any desired water depth.

Another significant aspect of the tower invention is the shell of crossbracings which join through unitary crosses being uniformly dimensionedthroughout the tower structure which serve to transmit loads through thecrossing junctions rather than into the tower legs. The junctions may befabricated in large quantities by mass production techniques since thedimensions of the junctions are uniform throughout the tower design.Therefore, fabrication time and labor are minimized.

Another significant aspect of the invention is the provision of largediameter pilings juxtaposed peripherally around the base of the offshoretower to withstand large shear and bending loads while permitting thevery long jacket legs to remain economically slender.

A further significant aspect of the invention is the provision of therotating pile guiding and driving truss which serves to accurately alignand drive a plurality of piles with a minimum amount of time and laborspent in pulling and setting pile strings.

A further significant advantage of the offshore tower is the readyadaptability of the design to vary the number of drilling locationsabout the periphery by the addition of guide collars and conductorswhich may be pinned generally parallel with existing jacket legs througha plurality of girder rings. In the alternative and where large baseloads are anticipated, jacket legs may be fabricated partially from thebed of the body of water upwardly along the surface of the towercoextensive with an increased number of cross bracings to add strengthto the base of the tower and to receive conductors within the interiorthereof in a protective manner.

Further, girder rings are positioned throughout the tower design havinga radiating bicycle bracing system 16 which supports the jacket legs andthe cross bracings from out of round deformation.

A significant method aspect of the invention is the provision of a novelmethod of fabricating an offshore tower of indefinite size and lengthsuitable to meet the increasing demands for larger offshore towers.

A further significant aspect of the invention is the manner of launchingthe thus constructed offshore tower within a body of water fortransportation to an offshore site. In this connection, lifting the baseof the tower and unloading the rail bearing guide permits a smooth andefficient launching operation.

An additional significant advantage is the utilization of the conductorsextending within the jacket legs in the dual function to guide adrilling string and as pilings.

While the invention has been described with reference to preferredembodiments, it will be appreciated by those skilled in the art thatadditions, deletions, modifications and substitutions, or other changesnot specifically described, may be made which will fall within thepurview of the appended claims.

What is claimed is:

l. A method of constructing an offshore tower operable to be positionedupon the bed of a body of water comprising the steps of:

fabricating a plurality of offshore tower girder ring assemblies, eachbeing fabricated by:

forming an outer substantially circular ring structure including atleast one substantially circular girder beam having a central axis;

positioning a hub coaxially about said central axis;

and

connecting a plurality of flexible members in tension radially betweensaid hub and said outer circular ring structure to reinforce said ringassemy;

suspending said fabricated girder ring assemblies in a mutually parallelposture along a horizontally disposed central axis wherein the planes ofsaid girder ring assemblies lie normal to said central axis by:

mounting said hubs upon generally vertical columns positioned betweenadjacent girder ring assemblies wherein the central axis of said girderring assemblies are horizontal and generally perpendicular with respectto said vertical columns, and

rigidly interconnecting the ends of adjacent hubs;

constructing tubular offshore tower legs about the perimeter of the ringassemblies by:

connecting hollow tubular offshore tower leg segments substantiallyperpendicularly between the planes of adjacent girder ring assemblies sothat said segments are disposed inwardly of said girder beam, and

connecting the ends of said tower leg segments with the ends of similartower leg segments positioned between the planes of adjacent girder ringassemblies to form continuous, axially aligned, hollow tubular towerlegs defining straight passages extending throughout the height of theoffshore tower capable of receiving conductors projected therethrough;

reinforcing said tubular offshore tower legs by:

constructing a shell of diagonally connected cross braces, andconnecting theconstructed shell to .said girder ring assemblies in suchmanner that said cross braces are located; inside of said girder 17 beamin non-intersecting relation to said tower method of constiuctipg anOffshore Power as 1 th t m l b f fined in claim 1 wherein said step offorming at least egs so a e iagona cross races rein orce one Circulargirder beam comprises;

said tubular tower legs without intersecting said forming an innergenerally circular beam having a tower legs f central axis;t i n l d borming an ou er genera y circu ar gir er earn coof constfucng anoffshore tower as a rially with respect to said inner generally circularfined in claim 1 wherein said step of reinforcing said girder beam;tower legs by constructing a shell of diagonal cross mterconneqmg saidInner and Outer Circular glrder b l d h beams with braces.

me u t 6 Step 0 10 4. A method of constructing an offshore tower asdeunitlng a unitary cross to the ends of said diagonal fined in claim 3and further comprising the step of:

cross braces at each intersecting junction of the Connectmg Sald W dsald Shell of qlagonal cross braces to said girder rings between saidinner cross braces to form a unction for transrnltting and Said Outerbeams loads through the intersection. I

1. A method of constructing an offshore tower operable to be positionedupon the bed of a body of water comprising the steps of: fabricating aplurality of offshore tower girder ring assemblies, each beingfabricated by: forming an outer substantIally circular ring structureincluding at least one substantially circular girder beam having acentral axis; positioning a hub coaxially about said central axis; andconnecting a plurality of flexible members in tension radially betweensaid hub and said outer circular ring structure to reinforce said ringassembly; suspending said fabricated girder ring assemblies in amutually parallel posture along a horizontally disposed central axiswherein the planes of said girder ring assemblies lie normal to saidcentral axis by: mounting said hubs upon generally vertical columnspositioned between adjacent girder ring assemblies wherein the centralaxis of said girder ring assemblies are horizontal and generallyperpendicular with respect to said vertical columns, and rigidlyinterconnecting the ends of adjacent hubs; constructing tubular offshoretower legs about the perimeter of the ring assemblies by: connectinghollow tubular offshore tower leg segments substantially perpendicularlybetween the planes of adjacent girder ring assemblies so that saidsegments are disposed inwardly of said girder beam, and connecting theends of said tower leg segments with the ends of similar tower legsegments positioned between the planes of adjacent girder ringassemblies to form continuous, axially aligned, hollow tubular towerlegs defining straight passages extending throughout the height of theoffshore tower capable of receiving conductors projected therethrough;reinforcing said tubular offshore tower legs by: constructing a shell ofdiagonally connected cross braces, and connecting the constructed shellto said girder ring assemblies in such manner that said cross braces arelocated inside of said girder beam in non-intersecting relation to saidtower legs so that the diagonal cross braces reinforce said tubulartower legs without intersecting said tower legs.
 2. A method ofconstructing an offshore tower as defined in claim 1 wherein said stepof reinforcing said tower legs by constructing a shell of diagonal crossbraces includes the step of: uniting a unitary cross to the ends of saiddiagonal cross braces at each intersecting junction of the cross bracesto form a junction for transmitting loads through the intersection.
 3. Amethod of constructing an offshore tower as defined in claim 1 whereinsaid step of forming at least one circular girder beam comprises:forming an inner generally circular beam having a central axis; formingan outer generally circular girder beam coaxially with respect to saidinner generally circular girder beam; interconnecting said inner andouter circular girder beams with braces.
 4. A method of constructing anoffshore tower as defined in claim 3 and further comprising the step of:connecting said tower legs and said shell of diagonal cross braces tosaid girder rings between said inner and said outer beams.